tac:tldr:3cf73

The tac command is used in Linux/Unix systems to reverse the order of lines in a given file(s). The -b option is not a valid option for the tac command, so assuming it's a placeholder meant to represent the actual filenames.

The command tac -b ${filename1 filename2 ---} likely intends to reverse the order of lines in multiple files. However, the placeholders ${filename1 filename2 ---} are not valid syntax.

To use the tac command properly, you should replace ${filename1 filename2 ---} with the actual filenames you want to reverse. The command would look like:

tac file1.txt file2.txt

This would reverse the order of lines in both file1.txt and file2.txt and display the result.